1. Field of the Invention
The present invention relates to a compatible optical pickup which is compatible with disks having different thicknesses, and more particularly, to an optical pickup enabling adjustment of a focus error signal offset, and reducing the focus error signal offset during recording/reproducing disks having different thicknesses.
2. Description of the Related Art
An optical pickup device is used for recording and reproducing an image, sound or data on and from an optical disk at a high density. The optical disk includes a substrate made of plastic or glass having a predetermined thickness, through which incident light passes, and an information recording layer formed behind the substrate. The size of the optical disk is standardized, and efforts are now concentrated on increasing the information recording density on the information recording layer, to record more information on the optical disk having a predetermined size.
A digital versatile disk (DVD) and high definition (HD)-DVD, released recently, are high-capacity optical disks capable of recording massive amounts of information. The thickness of the DVD is standardized to be different from that of a compact disk (CD), taking account of an allowance error in mechanical disk slant and numerical apertures of objective lenses.
That is, an optical pickup for recording/reproducing information on/from a CD has an objective lens having a numerical aperture of 0.45, while an optical pickup for recording/reproducing information on/from a DVD has an objective lens having a numerical aperture of 0.6, to increase recording/reproducing density. The thickness of the CD is 1.2 mm while the thickness of the DVD is 0.6 mm due to the allowance error of disk slant, caused by using the objective lens having a larger numerical aperture. It is likely that the thickness of the HD-DVD will be standardized to 0.6 mm.
Also, wavelengths of the light source for reproduction are different in the DVD and CD. That is, while the wavelength of the light source for reproduction of a CD is approximately 780 nm, the wavelength of the light source for reproduction of a DVD is approximately 650 nm. Also, for reproduction in the HD-DVD, a light source emitting light having a short wavelength of approximately 420 nm will be required.
Because of the difference of thickness between the CD and DVD, spherical aberration occurs when information is recorded/reproduced on/from a CD using an optical pickup for the DVD, so that enough light intensity for recording information cannot be obtained or a reproduced signal deteriorates.
Referring to FIG. 1, which shows the optical arrangement of a general optical pickup for DVD, the optical pickup includes a light source 1, a light beam splitter 7 for changing the path of incident light, a mirror 8 to reflect the incident light from the beam splitter, an objective lens 12 for converging incident light reflected from the mirror 8 to form a light spot on the recording surface of a disk 19, and a photodetector 18 for detecting an error signal and a radio frequency signal. Here, the light source 1 emits a light having a wavelength of approximately 650 nm, and the numerical aperture of the objective lens 12 is 0.6.
The light emitted from the light source 1 passes through the beam splitter 7 and is reflected by the mirror 8 to be condensed by the objective lens 12 and form a light spot on the recording surface of the disk 19.
Then, light reflected from the disk 19 passes through the objective lens 12, is reflected by the mirror 8, passes through the beam splitter 7 to be incident on the photodetector 18. As shown in FIG. 2, the photodetector 18 includes four light-receiving regions A, B, C and D which separately undergo photo-electric conversion to allow the detection of a focus error signal (FES) by an astigmatism method. Here, a distance d' between the light-receiving regions is less than approximately 10 .mu.m. In FIG. 1, reference numeral 3 represents a grating for diverging the incident light by diffraction to detect a tracking error signal by a three-beam method, and reference numeral 16 represents a condensing lens for condensing the incident light to form a light spot on the photodetector 18.
In operation, the beam splitter 7 is astigmatic, which affects the light reflected from the disk 19 as it passes through the beam splitter 7. Thus, a light spot formed on the photodetector 18 during the recording/reproduction of a DVD changes according to the distance between the objective lens 12 and the disk 19 as shown in FIGS. 3A through 3E. Here, FIGS. 3A and 3B show a light spot received by the photodetector 18 in the case where the distance between the objective lens 12 and the disk 19 is longer than the focal distance of the objective lens 12. FIG. 3C shows a light spot received by the photodetector 18 in the case where the distance between the objective lens 12 and the disk 19 is equal to the focal distance, that is, under an on-focus state, and FIGS. 3D and 3E show a light spot received by the photodetector 18 in the case where the distance between the objective lens 12 and the disk 19 is shorter than the focal distance.
In the optical pickup, the FES is obtained from the difference between the sums of signals of diagonally opposite light-receiving regions. That is, the FES is obtained by the difference between the sum of signals of the light-receiving regions A and C, and the sum of signals of the light-receiving regions B and D. In an on-focus state as shown in FIG. 3C, the value of a reproduced signal becomes the maximum, and the FES is approximately 0.
When recording/reproducing information on/from a CD using an optical pickup device for DVD, the light spot formed on the photodetector 18 is distorted due to the spherical aberration caused by the difference between the thickness of the DVD and CD. In this case, the FES is also distorted, so that focusing is difficult. Accordingly, it is difficult to record/reproduce information on/from a CD.
To overcome the above problems, an objective lens 12' having an annular light control pattern 13 is adopted as shown in FIG. 4, so that the FES as shown in FIG. 5 is obtained when recording/reproducing information on/from the CD. In FIG. 5, the horizontal axis represents the distance between the objective lens 12' and the disk 19, that is, the moving amount of the objective lens 12', and the vertical axis represents an FES.
The distance between the disk 19 and the objective lens 12' is controlled to achieve focus according to the FES, thereby recording/reproducing information on/from the CD.
However, even though the distance between the CD and the objective lens 12' is correct for focus, the FES is not equal to 0 but has a predetermined FES offset value due to the effect of the spherical aberration caused by the difference in thickness between the CD and the DVD. As shown in FIG. 5, the focus error signal offset is expressed as a percentage of the focus error signal value Wb in an on-focus state where the value of the reproduced signal is the maximum, with respect to the maximum amplitude Wa of the focus error signal. If the focus error signal offset is over 10%, compatibility with the CD is impossible. In particular, when reproducing information from the CD using an optical pickup for HD-DVD recording/reproduction, adopting a light source emitting light of approximately 420 nm and an objective lens having the numerical aperture of 0.6, the focus error signal offset becomes 30% due to the effect of the spherical aberration, so that it is impossible to reproduce information from the CD.